1. Technical Field
The present invention relates to a semiconductor device which contains a bipolar transistor.
2. Related Art
Semiconductor devices have a protective element for the purpose of protecting internal circuits from static electricity and so forth. The protective element is occasionally configured by using a bipolar transistor, typically as described in Japanese Laid-Open Patent Publication No. 2009-4763.
In a semiconductor device described in Japanese Laid-Open Patent Publication No. 2009-4763, the bipolar transistor has a double-layered base region. More specifically, a first base region connected to a contact is surrounded by a second base region. The second base region has an impurity concentration lower than that of the first base region.
Operations of the bipolar transistor will be outlined referring to FIG. 14. When voltage applied to a collector extracting region 222 reaches collector-base Avalanche breakdown voltage, breakdown occurs between the collector and the base 214. At this moment, current flows between the collector extracting region 222 and a base extracting region 218, through a sinker 210, a buried layer 208, and a deep well 212 (indicated by a broken line (OFF)).
If the current further increases, potential at a position right under an emitter region 220 rises by a level equivalent to (current×resistance component) ascribable to the resistance component of the first base region 214. The rise in potential causes turn-on of the junction between the emitter 220 and the base extracting region 218, and allows current to flow from the base extracting region 218 towards the emitter region 220. This triggers the bipolar transistor, and allows current to flow from the collector extracting region 222, through the sinker 210, the buried layer 208, the deep well 212 and the first base region 214, towards the emitter region 220 (indicated by a broken line (ON)).